Waveguides of the configuration, in which a high refractive index core portion is provided inside quartz or another vitreous material, already have been used in actual practice. In particular, thin silicon wire waveguides, in which a “thin silicon wire” made up of high refractive index silicon (Si) is used as the core, or defect waveguides (two-dimensional photonic crystal defect waveguides), in which two-dimensional photonic crystals are used, have recently attracted much interest and have been the subject of intensive research and development. The structure of a two-dimensional photonic crystal defect waveguide is described here. First of all, a two-dimensional photonic crystal having a two-dimensional periodic refractive index structure is formed, for example, by arranging regularly-spaced holes in a high refractive index thin-film layer made of Si. It should be noted that the two-dimensional photonic crystal is configured in such a manner that a complete photonic band gap in the operation frequency region is formed within a plane including a direction exhibiting refractive index periodicity (periodic refractive index direction). Furthermore, introducing line defects into this two-dimensional photonic crystal forms a defect waveguide. Light can propagate through the defect portion of such a defect waveguide, but cannot propagate through locations into which no defects have been introduced. Consequently, light entering the defect portion is confined in the defect portion and can propagate without leaking out.
Function devices utilizing the above-described waveguides include, for example, Bragg reflection devices. FIG. 16 is a plan view schematically illustrating Bragg reflection devices with a small refractive-index difference provided in a straight waveguide. In (a) of FIG. 16, a Bragg reflection portion 90 is provided by forming high refractive index portions 89 in a periodic fashion in a single location in the longitudinal direction of a waveguide 88. The Bragg reflection portion 90 then selectively reflects propagating light with a frequency corresponding to the period of the Bragg reflection portion 90. It should be noted that the high refractive index portions 89 can be formed using a well-known technique called “interference exposure”, in which ultraviolet (UV) light is employed. Moreover, if, as shown in (b) of FIG. 16, Bragg reflection portions 92 are provided in two locations in the longitudinal direction of a waveguide 91, then a Fabry-Perot resonator can be formed, which resonates light of specific frequency components in the portion (resonant portion) that lies between the pair of Bragg reflection portions 92.
Furthermore, also known is a Fabry-Perot resonator wherein, as shown in FIG. 17, Bragg reflection portions 95 having holes 94 formed in a periodic fashion are provided in two locations in the longitudinal direction of a semiconductor waveguide 93 (for example, see JP 2003-186068A).